Shoe with BG removed.
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Shoe with white BG layer added below shoe layer.
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Make a selection of the shoe.
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On a new layer below the shoe layer and above the BG layer, fill the selection with black. (Note: Just showing shadow layer with shoe layer off for demo)
(Additional note: You can also turn this layer into a smart object at this point, making the step of adding a Gaussian Blur editable!)
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Deselect. Enter free transform ( Cmd/Cntrl + t)
Transform the shadow to where you like it.
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Add Gaussian blur.
Lower opacity.
Can add a gradient via a layer mask if wanted.
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